Well, it's a 5/16-18 with a split washer, and depending on what water neck you are using, the length would be 1" - 1 1/2". Make sure the threads in the holes are clear. I like to use the OEM stainless Ford 190° thermostat, with the air bleed at the top, makes putting the coolant back in a snap. Glue it to the water neck with sealant, then the gasket. I use Permatex non hardening. Use it on the manifold surface, too.
The thermostat is stainless, not the housing. Of course, the last version of the early 289/302 housing was over $100, so Ford had a pretty high opinion of it.
Here's the thermostat, note the air bleed at the top: